The long-term goal of this application is to determine how the nervous system coordinates posture with arm movements. When man moves and interacts with the environment, the reaction forces that the environment exerts back on the body must be accommodated. These forces can change the posture of the body and, consequently, alter the accuracy of the movements. When movements themselves (and their possible interaction with the environment) are about to be executed, the nervous system acts to anticipate these reaction forces by contracting related postural muscles prior to contracting the muscles that will actually carry out the intended specific action. This postural activity has been termed "anticipatory postural adjustments", and it is essential for carrying out many normal, everyday movements. The investigators propose to carry out three sets of experiments to develop a better understanding of how the nervous system controls the posture of the body while it simultaneously moves the arm. Each set of experiments addresses a specific aim: (1) To examine how kinesthetic input is used to determine how long to wait after anticipatory postural activity begins to trigger arm movements in an unfamiliar mechanical environment. This will involve subjects who are neurologically intact and subjects with multiple sclerosis. (2) To determine how the nervous system learns to coordinate posture and arm movements in an unfamiliar mechanical environment. This will involve only neurologically intact subjects. (3) To examine how the nervous system organizes muscle activity during targeted, and presumably purposeful, arm movements (only normal subjects will participate). It is claimed that these experiments will not only shed light on relatively unexplored areas of movement control, but may also lead to better treatment of some motor disorders, controlling limb prostheses, as well as moving accurately in the microgravity of outer space.